From a study in the journal Nature Medicine,
"orally dosed l-carnitine is metabolized by intestinal bacteria to produce a substance called trimethylamine (TMA), which is further metabolized into trimethylamine-N-oxide (TMAO). It is theorized that TMAO prevents of excess cholesterol from being excreted, thus allowing it to infiltrate artery walls, which is being studied as a possible culprit for increased risk of cardiovascular disease."